The heavy metals (HMs) and metalloids such as Cr(VI), As(Ill), and Pb(II) in contaminated water are toxic even at trace levels and have caused devastating negative health impacts on human beings. Hence, the effective adsorption of these heavy metals from contaminated water is important to protect biodiversity, hydrosphere ecosystems, and human beings. In this study, a leachate modular tower (LMT) was developed for the singular purpose of adsorbing HMs. The LMT contained nano-slag as a liner, which was synthesized from slag. The nano-slag was blended in different proportions of 90:10; 80:20, 70:30, 60:40, and 50:50 to the combined mass of clay and nano-slag, to evaluate the most efficient ratio of the blends capable of adsorbing HMs and metalloids with 100% efficiency. A series of leachate tests were performed to evaluate the adsorption capacity of LMT with different embedded liners. Attenuation periods of 2, 5, 7, and 10 days with a temperature of 500 °C were also selected to improve the sorption rate and uptake of HMs. Subsequently, the effluents were subjected to inductive coupled plasma mass spectrometry (ICP-MS) tests to evaluate the concentrations and percentages of adsorbed HMs, which were calculated using a pseudo-first-order adsorption model. The results revealed that the removal of 98%As, 99%Cd, and 99.9% Pb was achieved with a 50%:50% ratio of soil and nano-slag as the liner at 10 days equilibrium period. Furthermore, 98%Zn, 95.45%Cu, 93.3%Fe, 97%Ni, and 89% Hg were achieved upon further investigation using the same dosage of soil and nano-slag and equilibrium conditions. The scanning electron microscopy (SEM) tests demonstrated that some traces of the absorbed HMs and metalloids were found on the liner surfaces, indicating significant changes in microstructure. The results indicated the sorption rate increased significantly due to the elevated temperature, aluminosilicate structure, and prolonged attenuation period, which are also associated with an elevated pH level and higher cation exchange capacity (CEC), of the liner. Doi: 10.28991/CEJ-2023-09-06-017 Full Text: PDF

The soil water retention curve (SWRC) or soil–water characteristic curve (SWCC) is a fundamental feature of unsaturated soil that simply shows the relationship between soil suction and water content (in terms of the degree of saturation and volumetric or gravimetric water content). In this study, the applications of the SWRC or SWCC have been extensively reviewed, taking about 403 previously published research studies into consideration. This was achieved on the basis of classification-based problems and application-based problems, which solve the widest array of geotechnical engineering problems relevant to and correlating with SWRC geo-structural behavior. At the end of the exercises, the SWRC geo-structural problem-solving scope, as covered in the theoretical framework, showed that soil type, soil parameter, measuring test, predictive technique, slope stability, bearing capacity, settlement, and seepage-based problems have been efficiently solved by proffering constitutive and artificial intelligence solutions to earthwork infrastructure; and identified matric suction as the most influential parameter. Finally, a summary of these research findings and key challenges and opportunities for future tentative research topics is proposed.

South Africa is cut-across an arid and semi-arid zone, this implies that 95% of its roadbed is under unsaturated condition. In this context, resilient performance of subgrades is determined considering matric suction ( $\psi$) and traffic loading parameters for resilient modulus (M_R) analysis through the application of unsaturated soil mechanics. Expansive natural subgrade soil sample collected from Northern Cape in South Africa is selected for this study. Series of M_R tests, Free swell index tests, Zero swelling tests, suction tests, X-Ray Diffraction and Scanning Electron Microscopy tests were conducted to investigate the unsaturated response of expansive subgrade stabilized with varying percentages 4%, 6%, 8% and 10% of activated fly ash (AFA). The suction components of the representative subgrade soils were determined through filter paper method. The soil-water retention curve (SWRC) was also analysed using Fredlund and Xing, and Seki’s models. The results showed that inclusions of 6%, 8% and 10% AFA caused some considerable increase in M_R and suction values to a limiting values 6% AFA content, below which resilient modulus values were reduced. The low M_R values are recorded in 4% AFA stabilized specimen, as a result of insufficient free reactive calcium oxide. This resulted to incomplete pozzolanic reaction with the clay minerals. Significant changes were observed in the soil pore structure, based on microstructure analysis of the stabilized subgrade soils. Inter-cluster cemented bonds, were noted on the surface structure of the stabilized subgrade compared to unstabilized subgrade soil, due to increase in M_R.